Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids, which are more easily transported and thus more economical. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is converted into a mixture of carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted into hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas, is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). Fischer-Tropsch synthesis generally entails contacting a stream of synthesis gas with a catalyst under temperature and pressure conditions that allow the synthesis gas to react and form hydrocarbons.
More specifically, the Fischer-Tropsch reaction is the catalytic hydrogenation of carbon monoxide to produce any of a variety of products ranging from methane to higher alkanes and aliphatic alcohols. It is the catalytic nature of the Fischer-Tropsch reaction that makes the process economically feasible. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification).
Catalyst systems often employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves one or more measures of the performance of a catalyst, such as activity, stability, selectivity, reducibility, or regenerability. For example, ruthenium, rhenium, and platinum are known to increase the reducibility of cobalt.
Further, in addition to the catalytic metal, a Fischer-Tropsch catalyst often includes a support material. The support is typically a porous carrier that provides mechanical strength and a high surface area in which the catalytic metal and any promoter(s) may be deposited. Catalyst supports for catalysts used in Fischer-Tropsch synthesis of hydrocarbons have typically been refractory oxides (e.g., silica, alumina, titania, zirconia or mixtures thereof).
After a period of time in operation, a catalyst may become deactivated, losing its effectiveness for catalyzing the desired reaction to a degree that makes the process uneconomical at best and inoperative at worst. The more deactivated a particular catalyst is, the less efficient the catalyst is at enhancing the rate of the desired reaction. At this point, the catalyst can be either replaced or regenerated. Replacement of catalyst could be quite expensive due to the loss of expensive metals and cost of making a replacement catalyst. For these reasons, regeneration is preferred over replacement.
Traditionally, regeneration methods for Fischer-Tropsch catalysts have used operating conditions similar to the Fischer-Tropsch operating conditions. However, this approach limits the scope of catalysts that can be effectively regenerated to those for which regeneration at operating conditions is possible, such as those deactivated by ammonia or hydrogen cyanide poisoning or to some extent, surface condensation of heavy wax products.
Hence, there is still a great need to identify new regeneration methods, particularly methods that can effectively regenerate Fischer-Tropsch catalysts without having to replace the catalysts and without significant downtime or loss of production. Also, there is a need for regeneration methods that can regenerate Fischer-Tropsch catalysts that have been deactivated by a variety of deactivation mechanisms.